1. Technical Field
This invention relates to covers for air bag units located within the interior of a motor vehicle and in particular relates to covers having a tear seam therein that is configured to define an uninterrupted outer surface on the cover. Further, the invention relates to an apparatus and a method for forming hidden tear seams in such covers for air bag units.
2. Description of the Related Art
Various proposals for covering air bag units have evolved. One primary objective is to provide a cover that has a weakened section therein that will separate upon air bag deployment to provide an opening in the cover through which the air bag can escape following vehicle impact that is sensed by a controller. A gas generator is ignited by the controller to produce gas for inflating the air bag as it is deployed.
Air bags are mounted in canisters that can be mounted in the steering wheel or in an instrument panel or crash pad on the driver's side of the vehicle. If the air bag is mounted on the passenger side, typically the canister is stowed behind an instrument panel either in a top mount position or in a front mount position. Such mounting arrangements require that the cover material of the steering wheel hub or the skin or cover of the instrument panel be specially configured during manufacture to produce a weakened section to define a seam therein that will readily open during bag deployment.
Such manufacturing methods and resultant structure must consider various functional and aesthetic requirements. The seam must be configured to separate such the air bag can be deployed immediately after vehicle impact. However, the cover must have a seam that is configured such that the air bag unit is protected against either accidental or intentional tampering and additionally the seam should be configured to meet long term weathering and interior design appearance requirements. As to design appearance, there is a desire in some cases to form the surface of the cover as a smooth uninterrupted surface so that there is no indication that there is an air bag unit disposed beneath the surface. This interior design objective is desired both from a human psychology standpoint and from a pure esthetics standpoint. In the case of the psychological considerations, the smooth surface effectively meets the adage of out of sight out of mind such that a passenger is not reminded of the possibility of deployment of an air bag unit. The interior design consideration enables smooth flowing surfaces to be incorporated in the cover material of an instrument panel on the passenger side thereof.
U.S. Pat. No. 3,640,546, issued Feb. 1, 1972 to D. S. Brawn, includes a cover for a air bag unit having two doors that are covered by an outer layer of material that is weakened along a joint line by reducing the thickness of the outer layer. Such thin sectioned seam lines can cause a resultant depression over time that will essentially outline the outlet from the canister for the air bag component of the air bag unit. The arrangement does not solve either the psychological awareness problem or the problem of retaining a smooth, flowing surface appearance.
U.S. Pat. No. 4,246,213, issued Jan. 20, 1981 to Y. Takamatsu et al., discloses a method for casting a V-shaped section at the tear seam. While the outer surface of the cover material is smooth the V-shaped configuration also reduces the thickness of the outer layer in an undesirable manner.
U.S. Pat. No. 5,082,310 discloses a cover for an air bag unit. The cover includes two doors that are hinged for pivotal movement during air bag deployment. The doors are covered by a layer of foam and the layer of foam is covered by an outer skin having a V-shape groove that is cut to weaken the skin.
One problem common to designs that cut or score a tear seam into an outer skin is that plastic materials especially, at cold temperatures, are prone to becoming brittle such that the cut or score line can induce cracking along the seam. At opposite temperature extremes typical skin cover material such as thermoplastic olefins or thermoplastic polyvinyl chloride become relatively soft and deformable such that they will stretch and remain unbroken during initial bag deployment. Another problem that can arise with cut or scored tear seams is that they become observable (read through outer surface) following extended periods of exposure to heat and sunlight.
Another specific disadvantage of the cover shown in the '310 patent and another cover shown in U.S. Pat. No. 4,886,630 is that they require that the outer skin be separately formed on a heated mold and then removed from the mold and cut before being co-molded to a foam layer in a subsequent molding operation.
The problems arising from separately forming a skin and then cutting the grooves therein are obviated by the present method that employs casting techniques disclosed in U.S. Pat. No. 4,623,503, issued Nov. 18, 1986 to E. Anetis et al., U.S. Pat. No. 4,562,025, issued Dec. 31, 1985 to John D. Gray and U.S. Pat. No. 4,925,151 issued May 15, 1990 to John D. Gray all of which are commonly assigned to the assignee of the present invention and each of which is hereby incorporated by reference.